An electromechanical transducer device that is used as an ultrasonic transducer device (also referred to as ultrasonic transducer) is used in, for example, a diagnostic apparatus for a tumor etc. in a human body by transmitting and receiving ultrasonic waves, which are acoustic waves.
In recent years, a capacitive electromechanical transducer device (capacitive micromachined ultrasonic transducer, CMUT) using a micromachining technique is being actively studied. This CMUT transmits and receives ultrasonic waves by using a vibrating membrane. Also, this CMUT has a wide frequency band of ultrasonic waves that can be transmitted and received (i.e., CMUT has wide-band characteristics). Ultrasonic diagnosis using this CMUT and hence having higher precision than that of a medical diagnostic modality in the past is receiving attention as a promising technique.
In general, imaging apparatuses using X-rays, ultrasonic waves, and magnetic resonance imaging (MRI) are frequently used in medical fields. Also, studies on an optical imaging apparatus that obtains in vivo information by causing light emitted from a light source such as a laser to propagate into an analyte such as a living body and detecting the propagation light are being actively promoted in medical fields. There is suggested photoacoustic tomography (PAT) as one of such optical imaging techniques.
PAT is a technique that irradiates an analyte with pulsed light generated from a light source, detects acoustic waves (representatively, ultrasonic waves) generated from living tissues absorbing energy of light propagating through and diffused in the analyte at a plurality of detection positions, analyzes signals of these acoustic waves, and visualizes information relating to optical characteristic values of the inside of the analyte. Accordingly, information relating to optical-characteristic-value distribution of the inside of the analyte, or more particularly to optical-energy-absorption-density distribution can be obtained.
In an electromechanical transducer device (also referred to as ultrasonic transducer device) including electromechanical transducer elements that are formed on a substrate, part of incident ultrasonic waves may interfere with reflection waves that are reflected by a back surface of the substrate (a surface opposite to a surface of the substrate with the electromechanical transducer elements formed) and generate noise.
This noise problem has been recognized by certain degree in the past. Even with a technique of related art, as long as electromechanical transducer elements with a high-frequency region of several megahertz or higher (for example, 2 to 3 MHz or higher) are used, frequencies, which may cause noise, are high and are likely attenuated. Hence, the noise problem may be addressed by certain degree by providing an acoustic attenuating member on the back surface of the substrate. With a frequency that resonates within a substrate like PTL 1, noise can be reduced by certain degree by matching an acoustic impedance of the acoustic attenuating member to an acoustic impedance of the substrate. However, in the case of CMUT, since the frequency band is wide, the frequency band may contain ultrasonic waves with a frequency lower than 2 MHz. The ultrasonic waves with the frequency lower than 2 MHz are hardly attenuated, and easily pass through the substrate. Hence, the measure of related art only has a limited effect.
FIG. 5 shows a configuration of related art. In the configuration of related art (PTL 1), an acoustic attenuating member 14 is provided on a back surface of a substrate 12, and an electric signal is acquired from end portions of the substrate 12 through electric wiring 13.
The above-described ultrasonic transducer device used for the above-described ultrasonic diagnosis includes transducer elements that are two-dimensionally arrayed (arrayed in a plane) on a front surface of the substrate. For an array with a higher density, the transducer device has a structure in which the front surface and the back surface of the substrate are electrically connected and electric wiring is drawn from the back surface of the substrate. To acquire signals of the two-dimensionally arrayed electromechanical transducer elements, an electric wiring substrate has to be provided on the back surface of the substrate and the electric wiring substrate has to be electrically connected with the substrate. With this configuration, since the distance between the substrate and the electric wiring substrate is small, the acoustic attenuation on the back surface of the substrate results in that the reflection waves from the back surface of the substrate and the electric wiring substrate affects the electromechanical transducer elements, and hence a signal-to-noise (S/N) ratio is degraded. Particularly in a frequency band with 1 MHz or lower, wavelengths are large and attenuation is small. The influence becomes noticeable. Also, to reduce noise crosstalk, there is a method in which an electric wiring substrate or an integrated circuit is arranged on the back surface of the substrate, and the electric wiring substrate or the integrated circuit is electrically connected with the back surface of the substrate. At this time, the distance between the back surface of the substrate and the electric wiring substrate or the integrated circuit is as small as several hundred micrometers. Hence, even if the acoustic attenuating member of related art is provided on the back surface of the substrate, low-frequency acoustic waves easily reach the electric wiring substrate, and reflection waves may become noise.
PTL 2 describes that projections and depressions are formed on a back surface of an electric wiring substrate to reduce reflection waves. However, to attenuate acoustic waves with wavelengths larger than a predetermined value (acoustic waves with frequencies lower than 2 MHz), large projections and depressions are required. At the same time, the thickness of the electric wiring substrate is limited in a fabrication process and a soldering and mounting process.